FIG. 1 illustrates an example of a conventional control circuit for a passive OLED panel. As shown in FIG. 1, a circuit 10 includes n OLEDs OLED1-OLEDn, which are placed in channels 11-1n, respectively, and the conduction of the channels 11-1n are controlled by corresponding row signals RS1-RSn. In the passive OLED panel, the raw signals RS1-RSn turn ON the channels in turn so as to form a perceivable image by vision persistence. The brightness of the OLEDs OLED1-OLEDn correspond to the current amounts flowing in the channels 11-1n, respectively. Each of the channels 11-1n is controlled by a corresponding digital to analog converter DAC1-DACn. For simplicity, each of the digital to analog converter circuits DAC1-DACn is shown to be connected to only one OLED in FIG. 1, but more than one OLEDs may be connected to each of the digital to analog converter circuits in a real panel. The digital to analog converter circuits DAC1-DACn may be the simple type shown in FIG. 2 or the cascoded type shown in FIG. 3.
Specifically, the brightness of each of the OLEDs OLED1-OLEDn is controlled in the following manner. The current amount of a current source CS is proportionally mirrored into the digital to analog converter circuits DAC1-DACn by current mirrors consisting of a transistor Q and transistors in the respective digital to analog converter circuits DAC1-DACn. A digital switch control signal SW (also called as column signal or segment signal) determines which transistors in the digital to analog converters should be conducted. For example, as shown in FIG. 2, the conduction current amounts in the transistors may be designed as 1×, 2×, 4×, and 8×, respectively, and accordingly a 16-level brightness can be generated based on the switch control signal SW.
As sizes of OLED panels increase, the number of OLEDs used therein increase. As a consequence, the number of channels of OLEDs in one OLED panel may be more than several hundred or even up to several thousand. Such increasing number of channels causes a problem that current channels can not be easily matched with one another in current amount, and accordingly the brightness across the panel is not uniform. Such non-uniformness can sometimes be perceivable, in a worse case. One approach for solving such a problem is to provide respective matching control circuits dedicated to respective channels and to sequentially conduct a matching check and calibration procedure by sample-and-hold for every channel. However, such an approach is not practical because it requires an enormous sophisticated circuit and a time-consuming check procedure. Therefore, a circuit and a method to efficiently match the plurality of channels with one another in current amount are desired.